Breast cancer is the most common malignant tumor of females in the western hemisphere, with 178,500 new cases expected for 2007 in the U.S. alone. A growing body of evidence suggests that breast cancer growth is sustained by rare self-renewing cells, named breast cancer stem cells or breast cancer-initiating cells (BCIC), which are biologically distinct from their more numerous differentiated progeny. Although the design of therapies that eradicate BCIC has the potential to revolutionize the treatment of breast cancer, the identification of molecular therapeutic targets in BCIC isolated from human primary breast cancer specimens is an extremely difficult task. Several groups, including ours, have recently identified a sub-population of cancer-initiating cells (CIC) in several cell lines, including human breast carcinoma cell lines, opening the possibility to study BCIC in a readily available and abundant biological material. Specifically, we have found that mammary carcinoma, glioblastoma, melanoma cell lines and their single-cell clones contain cells capable of forming spheroids upon in vitro growth under stem cell-like conditions. Spheroid-forming cells displayed a gene expression profile consistent with CIC, increased tumorigenicity, and an altered pattern of chemosensitivity. We have also isolated by limiting dilution from the human breast carcinoma cell line MCF-7 the highly tumorigenic MCF-7/1-eGFP clone that expresses BCIC-associated surface markers. Based on these findings, we propose the long-term goal to design therapeutic strategies that preferentially target BCIC. The specific hypothesis is that we can design specific anti-BCIC therapeutic strategies directed against the products of genes over-expressed in human BCIC- enriched cell lines. First, we will develop BCIC models from human breast cancer cell lines. Then, we will identify genes over-expressed in BCIC by gene expression profiling. Finally, we will validate genes over-expressed in BCIC as breast cancer therapeutic targets. PUBLIC HEALTH REVELANCE: Breast cancer is the most common malignant tumor of females in the western hemisphere, with 178,500 new cases expected for 2007 in the U.S. alone. A growing body of evidence suggests that breast cancer growth is sustained by rare self-renewing cells, named breast cancer stem cells or breast cancer-initiating cells (BCIC), which are biologically distinct from their more numerous differentiated progeny. We propose to identify genes preferentially expressed in BCIC and to validate them as potential specific targets for breast cancer therapy. [unreadable] [unreadable] [unreadable]